Abstract:

Method and device for modulating a signal comprising data symbols and
reference symbols, characterized in that it comprises at least one step
(3) wherein semi-pilot symbols are introduced that transport less
information than the symbols customarily used but enough to obtain
decisions decided during a decoding step (9), the semi-pilot symbols
being disposed between the data symbols and the reference symbols.

Claims:

1-9. (canceled)

10. A method of demodulating a signal having data symbols and reference
symbols, comprising steps of using semi-pilot symbols that transport less
information than symbols customarily used but which allow a demodulator
better decision-taking for its decided estimations, before a decoding
step.

11. The demodulating method as claimed in claim 10, comprising a step of
inserting semi-pilot symbols between the data symbols and the reference
symbols, when the data sources are of different value or importance, the
semi-pilot symbols then transmitting the information that is most
protected.

12. The method as claimed in claim 10, wherein the semi-pilot symbols are
disposed at the end of the frame of the signal.

13. The method as claimed in claim 10, wherein for the semi-pilot symbols
a sub-constellation of the constellation used for the modulation is used.

14. The method as claimed in claim 10, wherein BPSK symbols are used as
semi-pilot symbols when QPSK modulation is employed.

15. The method as claimed in claim 10, wherein QPSK symbols are used as
semi-pilot symbols when 8 PSK modulation is employed.

16. The method as claimed in claim 10, wherein QPSK symbols are used when
16 QAM modulation is employed.

17. The method as claimed in claim 10, wherein semi-pilot symbols arising
from a different constellation are used as semi-pilot symbols.

18. A device for demodulating a signal comprising data symbols and
reference symbols, wherein it comprises at least one demodulator suitable
for implementing the steps of the method as claimed in claim 10.

Description:

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001]The present Application is based on International Application No.
PCT/EP2006/066233, filed on Sep. 11, 2006, which in turn corresponds to
French Application No. 05 09219 filed on Sep. 9, 2005, and priority is
hereby claimed under 35 USC §119 based on these applications. Each
of these applications are hereby incorporated by reference in their
entirety into the present application.

FIELD OF THE INVENTION

[0002]The invention relates notably to a method and a modem making it
possible to optimize decoding by inserting symbols of intermediate
quality. These intermediate quality symbols are called "semi-pilot
symbols" in the description which follows.

BACKGROUND OF THE INVENTION

[0003]The invention applies notably in the field of satellite
transmissions. It can also be used in other cases of employment such as
sporadic burst transmissions in a multi-path channel. The use of hybrid
preamble (reference+subset) makes it possible notably to substantially
lengthen the preamble while improving the decoding of the bits
transported by the hybrid preamble.

[0004]The invention is also used for continuous transmission modes.

[0005]The invention can also be beneficial in the case of fast-evolving
channels. Specifically, these channels require fast tracking of the
channel, which is conventionally obtained by repeating the preambles with
a small period.

[0006]The use of powerful codes, such as turbo codes, makes it possible to
reduce the values of the signal-to-noise ratio denoted by the
abbreviation SNR that are necessary for transmitting a message. This
reduction brings demodulators ever nearer to the limits of their channel
estimation capability. This is all the more true as the decrease in SNR
allows a rise in spectral efficiency related to the use of more complex
constellations, for example, the 16 QAM constellation (the abbreviation
standing for quadrature amplitude modulation), the 32 APSK constellation
(the abbreviation standing for amplitude phase shift keying), etc. The
coding schemes obtained, often BICMs (the abbreviation standing for Bit
Interleaved Coded Modulation) make it possible to get still closer to
Shannon's theoretical limit, assuming that the modem behaves optimally.

[0007]The demodulator must estimate parameters such as the arrival
amplitude, the phase, the incident frequency, the multi-path channel,
etc. This is all the more difficult as the noise increases.

[0008]The currently known methods for solving this problem consist, for
example, in periodically adding references at the signal emission level.
These references are disposed in blocks or in a discrete manner.

[0009]When the demodulation conditions are difficult, such as for example
for demodulations in short bursts (burst, TDMA) or when the channel is
difficult to demodulate (multi-path channel, considerable phase noise,
considerable frequency error or Doppler acceleration), the emission
periodically inserts, notably at the start (burst satellite) or in the
middle of a burst (GSM or global system mobile), known symbols, called
pilot symbols or reference symbols. The insertion of these reference
symbols increases the band used and the ratio Eb/NO corresponding to "the
energy per information bit transmitted" and "the white noise spectral
density".

[0010]When the reference symbols are not sufficiently numerous, the
demodulator makes estimation errors which result in noise added to the
noise received by the modem. This demodulation noise can degrade the
performance of the demodulator or even lead to a dropout of the latter
which no error code can remedy. Conversely, when the reference symbols
are too numerous there is a loss in terms of spectral efficiency (the
inserted symbols do not carry any information) and signal-to-noise ratio
(the energy invested in the reference symbols is not used by the
decoder).

[0011]One of the drawbacks of these techniques is therefore that they make
an important difference between the data symbols and the reference
symbols.

SUMMARY OF THE INVENTION

[0012]The idea of the invention consists notably in introducing symbols
"of intermediate quality" or semi-pilot symbols for example between the
data symbols and the reference symbols. The expression intermediate
quality is understood to mean notably symbols which transport less
information than the information symbols conventionally used, but which
allow the demodulator better decision-taking for its decided estimations,
DD (Decision Directed). These symbols are therefore called semi-pilots by
analogy with the pilot symbols which are fully known (so-called reference
symbols). It is for example possible to replace a group of R reference
symbols and of D data symbols by (R+D) semi-pilot symbols. The modem can
then use information of better quality for its DD estimation.

[0013]The energy transmitted to the semi-pilot symbols is equivalent to
that of the information symbols, but the free distance of the
constellation of semi-pilot symbols, that is to say the minimum distance
between two distinct points of the constellation is substantially more
considerable than for the native constellation. Usually the ratio of the
distances is chosen with a factor of at least root 2, i.e. 3 dB in terms
of signal-to-noise ratio.

[0014]In particular, a method of demodulating a signal according to the
invention, comprising data symbols and reference symbols, comprising at
least one step wherein use is made of semi-pilot symbols that transport
less information than the symbols that are customarily used but enough to
obtain weighted decisions of better quality and used by estimators of LMS
type, these semi-pilot symbols transporting bits whose better quality
benefits the weighted decoder which follows the demodulator.

[0015]The method comprises, for example, at least one step wherein use is
made of semi-pilot symbols that transport less information than the
symbols customarily used but wherein these symbols benefit from a
specific coding that is robust enough to obtain decisions without error
during a decoding step, the semi-pilot symbols being disposed between the
data symbols and the reference symbols. In another step, the result of
the decoding is remodulated, and the symbols thus obtained are used as
hard reference.

[0016]The higher quality of the simple semi-pilots makes it possible,
notably, to instigate the first iteration of an iterative demodulation
step, that is to say coupled to an error corrector code (usually a turbo
code or an LDPC), the semi-pilot symbols being disposed between the data
symbols and the reference symbols).

[0017]There may be error corrector code unit (usually a turbo code or an
LDPC between the information carried by the pilot symbols and the
semi-pilot symbols).

[0018]The information transported benefits from an unequal protection of
the information, that is to say this information is distributed into
groups of unequal interest, the most important bits having to benefit an
error rate that is lower by at least one order of magnitude. On emission
the most important bits are, for example, associated with semi-pilot
symbols and coded by a different coding from the other groups. In one
step, these bits/symbols are, for example, demodulated and decoded. In a
following step, the decoded information, benefiting from the maximum
protection, is remodulated to serve as reference symbols for the native
symbols. When the decoding is exact, the remodulated symbols have become
genuine references. When the decoding is erroneous, the information of
the less protected groups is no longer relevant and the use of erroneous
symbols is not detrimental to the message which is already lost.

[0019]The technique implemented by the invention makes it possible notably
to re-synchronize at lesser expense an equalization algorithm of the DFE
type (the abbreviation standing for Decision Feedback Equalizer). The use
of semi-pilot symbols allows locally correct decision-taking with about
half as many reference symbols.

[0020]Still other objects and advantages of the present invention will
become readily apparent to those skilled in the art from the following
detailed description, wherein the preferred embodiments of the invention
are shown and described, simply by way of illustration of the best mode
contemplated of carrying out the invention. As will be realized, the
invention is capable of other and different embodiments, and its several
details are capable of modifications in various obvious aspects, all
without departing from the invention. Accordingly, the drawings and
description thereof are to be regarded as illustrative in nature, and not
as restrictive.

BRIEF DESCRIPTION OF THE DRAWINGS

[0021]The present invention is illustrated by way of example, and not by
limitation, in the figures of the accompanying drawings, wherein elements
having the same reference numeral designations represent like elements
throughout and wherein:

[0022]FIG. 1 a diagram of an emission reception system comprising a
modulation device on emission and a demodulation device for reception,

[0024]FIGS. 3A to 3D, various choices for the constellations and the
semi-pilot symbols chosen according to the invention,

[0025]FIG. 4 a frequency cost function compared for an 8 PSK modem using
only reference symbols and for a modem using reference symbols and QPSK,

[0026]FIG. 5 a frequency cost function compared for a references and QPSK
8 PSK modem and for a modem using references, BPSK and QPSK (mixture),

[0027]FIG. 6 a Zoom of the cost function in the zone of validity of the
Cramer Rao bound.

[0028]The idea of the present invention consists notably, at the emission
step level, in introducing semi-pilot symbols between data symbols and
reference symbols and at the reception step level in using these symbols
of intermediate quality to optimize decision-taking at the demodulator
level. Certain fully defined pilot symbols are then replaced with
semi-pilot symbols. Since the semi-pilot symbols transport information,
it is possible (for a constant number of transmitted symbols) to
transform certain information symbols into semi-pilot symbols.

[0029]Shown diagrammatically in FIG. 1 is an emission-reception set. The
emission part comprises notably an interface 1 with the digital signal to
be emitted (signal composed of reference symbols and data symbols
originating from one or more sources). The digital signal is transmitted
to a coder 2, then to a framing device 3 whose function is notably to
introduce the symbols of intermediate quality or semi-pilot symbols. The
frame thus obtained is transmitted to a modulator 4. The modulated signal
carrying the symbols of intermediate quality is thereafter transmitted to
a radio emitter 5. Without departing from the scope of the invention, the
insertion of semi-pilot symbols can be performed by the set consisting of
the framing device and modulator.

[0030]The modulated signal after passing through the propagation channel
is received by a radio receiver 6 then demodulated by a demodulator 7
adapted to utilize the semi-pilot symbols, before being "demapped" 8,
then decoded 9. The decoded signal is transmitted to an interface 10.

[0031]The device then provides a method of unequal protection of
information by adapting the modulation to the relative resistance aimed
at for the various data streams.

[0032]During transmission, defects such as noise are added to the signal
emitted.

[0033]The number of these symbols and the way in which they are
distributed temporally in the frame of the signal at the coding level are
chosen, for example, as a function of the application aimed at. An
example is given in FIG. 2.

[0034]The method according to the invention comprises notably a step of
inserting semi-pilot symbols between the data symbols and the reference
symbols, when the data sources are of different value or importance. The
semi-pilot symbols then transmit the information that is most protected.

[0035]FIGS. 3A to 3D represent respectively various choices of
sub-constellation for various constellations, and a choice of
constellation of semi-pilot symbols that are not situated on the native
constellation of the information symbols.

[0036]A list of constellations and sub-constellations that can be used for
the implementation of the invention is given by way of nonlimiting
example. It is possible to choose, for example, a sub-constellation of
the constellation used: [0037]BPSK symbols {+1, -1}, when QPSK
modulation is employed {+1, + 1, -1, - 1-1}, [0038]QPSK symbols {+1, +i,
-1, -i}, when 8 PSK modulation is employed: exp{ -1*k*π/4;
0≦k<8}, [0039]QPSK symbols {+3+i, -1+3i, -3-i, 1-i} when 16 QAM
modulation is employed {+a+b+i; a=-3, -1, +1, +3} and b={-3, -1, +1,
+3}.It is also possible: [0040]to repeat repeated symbols when BPSK
modulation is employed {+1, -1}, [0041]to use another constellation which
is not a sub-constellation of the mother constellation used for
modulating the signal.

[0042]FIGS. 3A to 3D represent examples of use of the modem and the
obtaining of semi-pilot symbols on the basis of the reference symbols and
data symbols. The example is aimed, for the modem, at minimizing the
cost, that is to say the mean square error between the signal received
and the signal decided.

[0043]In FIG. 3A, the reference symbols, the data symbols and the
semi-pilot symbols obtained have been represented for QPSK modulation.

[0044]FIG. 3B shows diagrammatically for 8 PSK modulation two
possibilities of semi-pilot symbols obtained on the basis of reference
symbols and on the basis of the data symbols. Shown diagrammatically is a
weak QPSK semi-pilot, increasing the free distance by +5.3 dB and a
semi-pilot for BPSK, gaining +8.3 dB.

[0045]The example is given for a method in which the transmission uses an
8 PSK modulation associated with a turbo code of rate 2/3. Under these
conditions the association of modulation+code demands a signal-to-noise
ratio per information bit transmitted Eb/N0 equal to about 6 dB.

[0046]The 424 bits are therefore coded, within the framework of a BICM
modulation, as 636 bits which require 212 information symbols coded by 8
PSK.

[0047]Here the modem adds, for its own synchronization requirements, 32
reference symbols in the form 18 at the start of a packet, 6 at the end
of a packet and 6 uniformly distributed in the frame. It therefore
transmits 242 symbols.

[0048]The method according to the invention transforms the 6 postamble
symbols as well as the 12 information symbols modulated by 8 PSK
preceding these 6 postamble symbols into 18 information symbols modulated
by QPSK, therefore transporting the same number of coded bits. These 18
symbols are, in this example, placed at the end of a packet, to increase
the discriminatory ability in regard to the phase rotation at the end of
a packet and therefore to the frequency error.

[0049]FIG. 3C shows diagrammatically the case of the 16 QAM modulation,
with the reference symbol, the data symbol, the points of the semi-pilot
constellation shown solid, and the points of the mother constellation
which are not used and which appear dotted.

[0050]Another case for the 16 QAM modulation is represented in FIG. 3D.
There are 2 types of constellation for the semi-pilot symbols:

The set A optimizes the information passed to the decoder while the set B
of semi-symbols optimizes the Euclidean distance for a given energy. The
set B therefore optimizes the demodulation.

[0051]The gain for the modem is illustrated in FIG. 4. The cost versus
frequency curve for the "references only" system is below the
"references+QPSK" curve.

[0052]It is therefore apparent that the modem using the information that
the final symbols are QPSK-modulated will have a better estimate in terms
of frequency than a modem receiving a burst modulated solely with
references and 8 QPSK symbols.

[0053]The noise added by the modem during demodulation is therefore weaker
in the references=QPSK case and therefore, for equivalent code, the work
of the decoder will thereby be improved.

[0054]Additionally, during the symbol to bits conversion, better known by
the term demapping of the 18 QPSK symbols, the demodulator uses the
largest free distance of the QPSK constellation to provide better a
priori information on 24 bits of the corresponding 36 bits. This better
knowledge results in a gain in regard to the operating point of the
decoder. In the present case, the measured gain is 0.3 dB, i.e. a
signal-to-noise ratio C/N of 9 dB for an error rate of 10-4 instead
of 9.3 dB for the model with conventional references.

[0055]The results of the figure have been obtained by transforming two
QPSK symbols into pilot signals consisting of an 8 PSK symbol and of a
PSK symbol. In the figure it may be noted that the "Ref+QPSK" curve drops
in the vicinity of a rotation of π/2 on the burst, created by the
π/2 ambiguity of the QPSK symbols has been attenuated by the use of
BPSK symbols, which do not suffer this ambiguity.

This transformation benefits the modem (fewer risks of frequency error),
but is detrimental to the error corrector code since the latter then sees
three 8 PSK bits and a BPSK bit, which carry less information than 4 bits
arising from two QPSK symbols.

[0056]The "ref+QPSK" and "Ref+BPSK+QPSK" diagrams give the same
performance here.

[0057]FIGS. 5 and 6 show the gain afforded by the system according to the
invention within the framework of the packet transmission of ATM
(Asynchronous Transfer Management) cells of 424 bits. These figures
indicate the cost function for the frequency error made by the modem.
These cost curves are the image of the square error for the zero
frequency. An abscissa of x indicates a frequency error corresponding to
a rotation of x revolutions on the burst length.

[0058]It will be readily seen by one of ordinary skill in the art that the
present invention fulfils all of the objects set forth above. After
reading the foregoing specification, one of ordinary skill in the art
will be able to affect various changes, substitutions of equivalents and
various aspects of the invention as broadly disclosed herein. It is
therefore intended that the protection granted hereon be limited only by
definition contained in the appended claims and equivalents thereof.